In the case of a particle flow in a fluid which is to be measured, it can in fact be a question of solid or liquid particles, which are embedded in a gas or in another liquid and moved with the same or which are moved together. Measurements of this type are generally carried out in order to determine the particle size distribution and particle concentration in the fluid and can e.g. be used in space, exhaust air and medical technology. The detection of particles takes place through the stray light intensity scattered by a particle on a receiver, in that the particle flow is irradiated with suitable electromagnetic radiation in the infrared to ultraviolet light range and preferably with white light. Part of the stray radiation is detected by a detector. The particle characteristics which are of interest are determined by means of count pulses, which are evaluated by an evaluation electronics of a suitable nature positioned downstream of the detector.
Such methods and apparatuses are known from DE 196 12 569 C1 and U.S. Pat. No. 5,815,265. For increasing the dynamic range when checking the size distribution of particles in fluids, DE 196 12 569 C1 discloses the provision of two succeeding measuring volumes in a measuring channel through which a fluid flows. The measuring volumes are illuminated by means of a single light source, whose radiation is split by means of an arrangement of at least in part partly reflecting mirrors and lenses into two beam paths. Two photomultipliers are used for detecting the proportional scattered radiation. The illuminating device, the measuring arrangement and the detector arrangement form a constructionally compact unit, which has both optomechanical and electronic components.
In order to avoid particle detection errors in marginal areas of a measuring volume, U.S. Pat. No. 5,815,265 discloses the provision of specially shaped diaphragms in the illumination and stray light beam path. Apart from optomechanical elements, there are once again electronic components, particularly a light source and a light detector, which are constructionally connected to the measuring arrangement in a not further defined manner.
Thus, it is considered disadvantageous in the known methods and apparatuses that as a result of the constructional proximity, particularly the combination of measuring points and electronic components, a use in explosion-endangered environments is virtually excluded, at least without adopting additional costly measures. As a result of the known, compact constructional forms, measurement at difficultly accessible locations, e.g. on high chimneys or the like when measuring waste gases is not possible, if the in part sensitive and expensive electronic components are not to be exposed to damaging meteorological influences and must also be readily accessible. In known systems where a single light source is used for illuminating several measuring volumes, due to intensity losses on the optical components used for beam division, an illumination of different measuring volumes with an identical intensity is generally not ensured, which can have a negative effect during the evaluation of the measured results.
Even for the same design series, different particle counters have the disadvantage of different equipment characteristics. In the case of filter testing with a particle counter it is necessary to have a scanner, which is used upstream of the particle counter. In addition, there is a need for long sampling tubes from the sampling location to the scanner. An important disadvantage is the particle losses in the tubes.
With irregularly shaped particles such as e.g. with quartz dust or salt aerosols the stray light becomes higher due to a higher reflection percentage. Therefore an excessive diameter is measured. Finally the known sensors have a considerable weight.
The problem of the invention is to provide a method and an apparatus with which there is an intense illumination with high efficiency and in particular an identical irradiation of a plurality of measuring volumes and therefore there is an optimum comparability of the corresponding measured results.